<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>xiaosanjaio</title>
    <style>
        .box{
            width: 100px;
            height: 100px;
            position: relative;
            background:skyblue;
        }
    .box::after{
                content: "";
                border: 7px solid transparent;
                border-left-color:skyblue; 
                position: absolute;
                right: -14px;
                top:10px;
    }
    </style>
</head>
<body>
    <div class="box">

    </div>
</body>
</html>